The present invention relates to a thermal master making device for perforating a thermosensitive stencil or similar thermosensitive medium with heat to thereby make a master and a thermal printer including the same.
A digital thermal printer is conventional that uses a thermosensitive stencil as a thermosensitive medium. The thermal printer includes a thermal head having a number of heat generating elements that are arranged in an array in the main scanning direction. The heat generating elements selectively generate heat in accordance with an image signal representative of a document image so as to perforate a stencil. The perforated stencil, or master, is wrapped around a print drum including a porous portion. A press roller or similar pressing member presses a paper sheet or similar recording medium against the master. As a result, ink fed from the inside of the print drum is transferred to the paper sheet via the porous portion of the print drum and the perforations of the stencil, printing an image on the paper sheet.
More specifically, a platen roller is rotated while pressing the master against the thermal head. While the platen roller conveys the master in the subscanning direction perpendicular to the main scanning direction, the heating elements repeatedly generate heat in accordance with the image signal to thereby perforate the stencil.
The base temperature of the thermal head, i.e., the temperature at which the head starts generating heat varies with the environment in which the printer is operated. A change in base temperature translates into a change in peak temperature which Joule heat generated by the heat generating elements is expected to reach, effecting the configuration of perforations. For example, if the base temperature rises, then the area exceeding the perforation threshold of a stencil and the perforation diameter increase. Conversely, the perforation diameter decreases in a low temperature range. Further, the thermal response of the stencil itself is dependent on the environment. The thermal response refers to a period of time necessary for the stencil to reach a threshold. Consequently, a change in ambient temperature results in a change in perforation condition and therefore effects the quality of a print.
High resolution, high-speed master making and space saving (including compact design and low cost) are required of a modern thermal master making device. In practice, there are required resolution of 600 dpi (dots per inch) for size A3, master making speed of 2 milliseconds per line higher than the conventional 3 milliseconds per line, and the size reduction of a thermal head. The size reduction of a thermal head leads to high yield and low cost.
The above requirements, however, cannot be met without further aggravating the ill effect of a heat accumulation characteristic particular to a thermal head and therefore without causing the perforation conditions to vary, as will be described more specifically later.
A relation between a thermal head featuring high resolution, high-speed master making and space saving and the heat accumulation characteristic will be described hereinafter. As for high resolution, when the resolution of a thermal head is simply increased from 400 dpi to 600 dpi for size A3, the number of heat generating elements to generate heat increases. Therefore, for given thermal response of a stencil, the amount of heat to be generated simply increases. Further, an increase in the resolution of a thermal head translates into a decrease in the size of the individual heat generating element. Therefore, to guarantee a required amount of heat, it is necessary to raise the peak of Joule heat for given drive conditions. It follows that for a given level of heat output form a thermal head itself, resolution increases the amount of heat to accumulate in the head if simply increased. The level of heat is determined by the surface area of an aluminum radiation plate.
When the master making speed is increased, not only the duration of current supply to the heat generating elements of a thermal head, but also the duration of interruption of current supply (release of heat). Also, a stencil must be conveyed at a higher speed with the result that heat transfer efficiency from the heat generating elements to the stencil is lowered. Consequently, high-speed master making needs higher Joule heat than low-speed master making and therefore increases the amount of heat to accumulate in the head.
As for space saving, a decrease in the size of a thermal head itself results in a decrease in the size of the aluminum radiation plate and therefore in the thermal capacity of the head, i.e., a period of time necessary for the base temperature to rise. This, coupled with the fact that the surface area of the radiation plate decreases, reduces the amount of heat to be released to the outside and thereby increases the amount of heat to accumulate in the head.
As stated above, a thermal head satisfying the previously stated conditions causes more heat to accumulate therein than conventional. We experimentally found that such heat aggravated a difference in perforation condition between the leading edge portion and the tailing edge portion of a single master, which has not been addressed to in the past. Particularly, when image data had a high print ratio in the main and subscanning directions, the perforation diameter became far greater than a designed value in the trailing edge portion of a master, resulting in offset.
Moreover, irregularity in the various portions of a thermal head effects perforations. It was experimentally found that in, e.g., a portion where the resistance of the head approached the lower limit away from a mean value, perforations formed by the heat generating elements joined each other in the subscanning direction and lowered the resistance of a master to repeated printing. This is because in the case of constant voltage drive the heat generating elements whose resistance is lower than the mean value generate more heat than the others. Likewise, in a portion where perforations were formed by a small amount of heat, perforations formed by the heat generating elements joined each other in the subscanning direction and also lowered the resistance of a master to repeated printing.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-90746 and 11-115145, U.S. Pat. Nos. 5,685,222, 5,809,879, and GB 2277904A and 2294906A.
It is therefore an object of the present invention to provide a thermal master making device capable of obviating a difference in perforation condition between the leading edge portion and the trailing edge portion of a master as well as offset and low resistance to repeated printing, and a thermal printer including the same.
It is another object of the present invention to provide a low cost, thermal master making device using a conventional construction as far as possible, and a thermal printer including the same.
In accordance with the present invention, a thermal master making device includes a thermal head having a plurality of heat generating elements arranged in an array in the main scanning direction. A thermosensitive medium is moved relative to the head in the subscanning direction perpendicular to the main scanning direction while pressing the medium against the head. The heat generating elements repeatedly generate heat in accordance with an image signal to thereby make a master. The master making device includes a sensor for sensing ambient temperature around the head, and a correcting circuit configured to correct the amount of heat to be generated by the head in accordance with the ambient temperature sensed by the sensor. The amount of heat is corrected on the basis of the ambient temperature during master making operation.
A thermal printer including the above-described thermal master making device is also disclosed.